1. Field of the Invention
This invention relates generally to a method and apparatus for detection of an RF signal and, more particularly, to a low current RF signal detection circuit for use in electronic tolling.
2. Discussion of the Related Art
As any seasoned traveler or commuter knows, toll roads, toll bridges and the like are a significant part of the highway transit system. In order to provide funds for the maintenance and upkeep of toll roads and bridges, users are required to pay a fee related to the use of these roads and bridges. Generally, at a toll area, a series of toll booths extend across a roadway, and users are obligated to pay the toll rate in order to pass. Alternately, in a toll road situation, travelers will receive toll cards at toll booths as they enter a toll area, and then will pay the required fee depending on the point of exit of the toll area. In either of these situations, the commuter is required to stop his or her vehicle either to receive the toll card or pay the toll fee. Sometimes, depending on traffic congestion, users must wait significant periods of time before arriving at the toll booths. As a result, toll road and toll bridge users are inconvenienced beyond that of the extraction of funds.
In order to alleviate the inconvenience to the commuter, and reduce the expense of the tolling system, and thus hopefully the toll, it is known to use electronic tolling in which tolling booths, as well as the personnel required to operate them, are substantially eliminated. Electronic tolling is a process by which communications between a transponder associated with a vehicle and a roadside reader make a record of the vehicle operator's use of the toll road or toll bridge. In these types of systems, the roadside reader will continuously emit an RF signal which will be received by the transponder of the vehicle when the vehicle enters the tolling area. Upon receipt of the transmitted signal from the roadside reader, the transponder of the vehicle will emit an encoded signal to the roadside reader such that the roadside reader can identify the vehicle. Upon identification of the vehicle, the roadside reader will initiate a billing sequence. Different protocols for determining and billing the toll are known in the art.
The transponder in the vehicle is battery powered, usually by a 3 volt lithium battery. Because of this, it is important to attempt to limit the power drain on the vehicle transponder to times when the transponder is receiving a valid signal from the roadside reader in order to conserve battery life, and thus the need to replace the battery. For this reason, the vehicle transponders will generally include a signal detection circuit which monitors RF frequency signals, and when a valid tolling signal is present, cause power to be delivered to the transmitter and data analysis circuitry associated with the vehicle transponder.
FIG. 1 shows a prior art signal detection circuit 10 which is applicable to be used in a vehicle transponder (not shown) in order to wake-up subsequent data analyzing circuitry and a transmitter (not shown) when a valid tolling signal is present. An antenna (not shown) associated with the vehicle transponder receives RF signals and applies the signals to a band pass filter (not shown). The detection circuit 10 receives the RF signals from the filter. The signals are rectified by a rectifying diode 12 such that the carrier is removed and only the modulation signals will reach a positive terminal of a comparator 14. A signal is applied to the comparator 14 on an input power line 16 from a data analyzer circuit (not shown) at an intermittent rate in order to periodically power up the comparator 14. Typically, this rate is at about a 10% duty cycle of a clock signal associated with the data analyzing circuit. The power signal is also applied as a DC bias through a resistor R.sub.1 along with the RF input signal to the positive terminal of the comparator 14. The power signal is further applied to a negative terminal of the comparator 14 through a resistor R.sub.2. A potentiometer 18 adjusts the sensitivity of the comparator 14. The sensitivity of the comparator 14 is generally set for a relatively strong signal level. A second diode 20 is provided to compensate for temperature differences, and is matched to the diode 12. A capacitor C.sub.1 is used as a by-pass filter. Once the voltage on the positive terminal of the comparator 14 falls below the voltage on the negative terminal of the comparator 14, a signal present signal is outputted from the comparator 14. Once the signal detection circuit 10 determines that a valid signal is present, the data analyzing circuit will wake up and will receive the input from the positive terminal input of the comparator 14 in order to determine the tolling information.
Although the detection circuit 10 offers a number of advantages for waking up the data analyzing circuit of the transponder of the vehicle, there is still room for improving this system. One problem with the circuit 10 is the increased costs associated with providing matched diodes. However, a more serious problem involves the variability in turn on level over temperature and false wake-ups associated with the sensitivity requirements. Both of these factors affect the battery life of the vehicle transponder. If the potentiometer 18 is adjusted to increase the sensitivity too much, the circuit 10 may indicate a signal is present on noise or weak RF interference and drain the battery. Likewise, too low of a sensitivity may result in no detection when a valid roadside transponder message is being sent.
What is needed is a signal detection circuit for use in electronic tolling systems which distinguishes between the desired RF transmission and other RF transmission in order to only wake up the data analyzing circuitry when a valid signal is present. It is therefore and object of the present invention to provide such a signal detection circuit.